We summarize the implementation of the Differential Phase (DP) mode at the Keck Interferometer. Multicolor phase measurements are potentially a powerful astrophysical probe -- and can allow direct detection of Roaster-type exoplanets from the ground. The Keck Interferometer will measure differential phases between H-K, and K-L bands to levels of 3 mrad or better. At JPL, we are engaged in development and testing of instrumentation that will enable these extremely sensitive measurements. First on-sky observations are expected to start in middle 2005. In this article we describe DP and other related techniques, provide an outline of the instrument and present results from preliminary laboratory experiments.